Compact discs ("CDs"), digital versatile discs ("DVDs") and other similar discs are commonly used to store large volumes of data in a compact, durable recording medium. Initial applications of such media were for audio recordings, but CDs are increasingly used as a medium for storing computer data as read only memory, commonly referred to as CD ROMs.
Traditional CDs are designed to store a single, unalterable set of data, i.e., the data cannot be changed once the disc is manufactured. In such standard CDs, the disc commonly comprises a substrate which carries a reflective coating and a protective layer over the reflective coating. The substrate is typically formed of a relatively clear plastic material most commonly poly(bisphenol-A carbonate) or polymethylmethacrylate (PMMA) and has a bottom surface and an upper surface. In use, the bottom surface is oriented toward the laser used to read the CD and the upper surface carries the reflective coating. The reflective coating is typically a single layer of aluminum, gold or other reflective metal. The protective layer is typically a curable polymeric resin, e.g., a UV-curable acrylic resin such as a mixture of trimethylol propane triacrylate and neopentyl glycol diacrylate.
DVDs share much in common with the structure of standard CDs, but are configured to store more data on a disc of about the same physical dimensions. One common type of DVD, referred to as a DVD10, is much like two thinner CDs attached to one another. Such DVD10 discs have a pair of relatively thin CD-type structures with the data encoded on one face of a substrate. The data sides of the two thin discs are oriented toward one another and the two discs are then bonded together using an adhesive.
The upper surface of the substrate has a series of physical deformations therein. Typically, these deformations comprise a series of pits arranged in a predetermined fashion in the upper surface of the substrate. These pits are coated with the reflective layer such that the lower face of the reflective layer conforms to the profile of the upper surface, forming a fairly precise reflective interface that follows the molded contours of the substrate.
In use, the disc will be positioned adjacent a light source which emits light at a predetermined wavelength or range of wavelengths. The light source is typically a coherent source, e.g., an infrared laser. A light detector is positioned at a precise location with respect to the light source. The beam of light from the light source is passed through the substrate and strikes the lower face of the reflective layer. This reflected light then passes back through the substrate. The position of the lower face of the reflective layer will vary, resulting in a change in the intensity of the light measured at the light detector. As a result, when the disc is moved laterally with respect to the light source, the variation in the intensity of the light detected by the light detector will generate a readable binary data stream.
Conventional CDs and DVDs do have some potential downfalls, though. The integrity of the data stream generated from the reflected light will depend in large part on the precision of the placement of the reflective layer with respect to the light detector. If the lower face of the reflective layer falls outside of an acceptable range of positions, the reading device may be unable to effectively read the data contained on the disc. This, in turn, can result in unreliable file transfers from a CD ROM or in loss of fidelity of sound reproduction in musical CDs, for example.
The changes in the disc which can lead to improper positioning of the reflective layer with respect to the light source and light detector can be considered as falling into two types. The first is related to "macro" deformations of the disc, such as warping of the disc as a whole. The second potential cause relates to "micro" deformation of the disc in a relatively localized area.
Macro deformations can result from a variety of causes, including mistreatment of the disc itself. One common cause of macro deformations is sharp changes in temperature of the disc. The materials used to form the disc typically have different coefficients of thermal expansion. For example, the protective layer formed on the upper surface of the reflective layer can have a significantly different coefficient of thermal expansion than does the substrate. As the compact disc undergoes significant temperature variations, one side of the disc may expand or contract more rapidly than the other side, causing the disc to curve or bow out.
On a "micro" level, localized heating of the disc can cause similar warping or deformation on a small, localized scale. As noted above, the light source used to read the data from such discs frequently falls in the infrared range. This beam of light will strike a fairly localized area of the disc at any given time. This infrared beam can induce significant localized heating in the area where the beam is striking the compact disc. This is unlikely to cause the entire disc to warp or change shape, but it can change the shape and some dimensions of the precisely molded pits that encode the data carried on the disc.
One of the limitations of conventional CD and DVD structures is the inability to record data on the disc once it has been manufactured. In conventional CDs, there is no way to physically deform the upper surface of the substrate to add additional data to the disc. For this reason, a number of approaches have been developed to provide recordable optical media. In most of these recordable media, the reflectance is varied by inducing a chemical or phase change in a specialized portion of the media in response to a recording signal. These media rely on the chemically induced changes in reflectance to generate the readable data stream rather than relying on physical deformations in the upper surface of the substrate.
The most common types of recordable media rely on two different varieties of chemical or phase changes to alter the reflectance of specified locations on the disc. In one approach, the medium has a layer of a photoreactive compound which changes when illuminated with a specific wavelength or set of wavelengths of light at a sufficient intensity. Typically, when the photoreactive compounds are activated by the activating wavelength of light, they will change reflectance in a different wavelength, such as by changing color in the visible spectrum or infrared. As a result, a different wavelength of light can be used to read the data on the disc by detecting the change in reflectance dictated by the differently colored areas.
The other common type of recordable media relies on the presence of a ferromagnetic layer to allow the user to repeatedly change the data carried by the optical medium. In such a structure, a thin layer of an amorphous ferromagnetic material is applied over the flat, relatively featureless upper surface of a substrate. The ferromagnetic material has a magnetizable axis which is generally perpendicular to the surface of the substrate. Typical materials for this recording medium are combinations of rare earth metals and transition metals, such as alloys of gadolinium, dysprosium, terbium or praseodymium with iron, cobalt or chromium. Examples of magneto-optic recording media utilizing such thin ferromagnetic layers are taught, for example, in U.S. Pat. No. 4,695,510 (Sawamura) and U.S. Pat. No. 5,633,746 (Sekiya), the teachings of both of which are incorporated herein by reference. Two specific compositions known in the art are terbium-iron-cobalt (TbFeCo) films and tellurium-germanium-antimony (TeGeSb) films.
In use, such ferromagnetic recordable optical media are placed in a specialized recording device. The ferromagnetic layer is typically heated in a fairly localized area utilizing a focused beam of infrared light or the like. This increase in temperature makes it easier for the crystals of the ferromagnetic material to reorient themselves in response to a strong magnetic field. The magnetic field adjacent the heated area of the disc can be changed to change the crystalline orientation of the material in one area as compared to the orientation in a different area. These different crystalline orientations result in different reflectance levels at the wavelength of light used in the CD reader. The data recorded in one recording session can typically be recorded over by again inducing localized heating of the ferromagnetic recording layer in the presence of a carefully controlled electrical field.